THE GEOLOGICAL TIME-SCALE 
ELEMENTARY GEOLOGY 
GROUP OR ERA. 
SYSTEM OR PERIOD. 
SERIES OR Epocu. 
CENOZOIC. Pleistocene.! .° . 4 s57e==s= 4 see ae ae 
Mammalian 
aie eo elbow Names of subdivisi 
Eocene. Undivisions 
MESOZOIC. Cretaceous. based mainly upon 
Reptilian Era. 
Juratrias.? 
the south and west. 
PALEOZOIC. 
Era of 
Invertebrates. 
Carboniferous. 
Permian.* 
mee ee a aa a aaa a ae ee ee eS 2 
Coal Measures. 
Subcarboniferous. 
Devonian. 
Chemung and Catskill. 
Oriskany. 
Silurian. 
Helderberg. 
Niagara. 
Ordovician. 
Cambrian 
Trenton. 
Georgian. 
ARCHEAN. 
Huronian.® 
Laurentian .® 
1 Pleistocene adopted in place of the older Quaternary. 
2 Neocene and Eocene used in place of Tertiary. 
83 In Europe, Triassic and Jurassic are recognized ; but the rocks of these 
periods in America have not been so well separated. In the east the Triassic 
rocks are known as Newark. 
4 Permian is given the rank of period in Europe. 
5 Nearly the same as Algonkian of the United States Geological Survey. 
6 The Fundamental Complex of the United States Geological Survey. 
